Electrical discharge machining (EDM) is based on the principle of erosion of metal by spark discharges. The spark comprises a transient electric discharge to the space between two charged electrodes which are called the tool (or moving electrode) and the workpiece. The discharge occurs when the potential difference between the tool and the workpiece is large enough to cause a breakdown in the intervening fluid medium to procure an electrically conductive spark channel. Spark initiation occurs in the order of one microsecond, followed by a rapid increase in plasma temperature to the order of 20,000.degree. F. and widening of the spark into a relatively thin flat disc. Heat transfer from the plasma to both the tool and the workpiece melts, partially vaporizes, and partially ionizes the metal in a thin surface layer.
EDM is often carried out in a moving, filtered dielectric fluid such as kerosene. The fluid in the work gap, (i.e. approximately 0.0005 to 0.005 inches) is always contaminated with spherical solidified particles of work and tool material which has just been eroded. It is thought that the presence of these metallic spheres produces relatively high conducting paths from tool to workpiece.
Because the spacing between the tool and workpiece is critical, the movable tool or electrode is commonly controlled by a servo mechanism.
The rate of metal removal depends to a large extent on the average current in the discharge circuit. It is also a function of the electrode characteristics, the electrical parameters and the nature of the dielectric fluid. The rate of metal removal is influenced by the electrical conductivity of the workpiece, but not by the hardness of the material. Thus, as long as the workpiece material is electrically conductive it can be satisfactorily cut. Materials that may be cut in such a fashion include not only metals such as high-speed steel, heat-treated tool steel and titanium, but also semi-conductors such as gallium, silicon and germanium and sandwich-type combinations such as sintered cutting material on a steel body, with or without a backing layer. The rate of removal is normally varied by changing the number of discharges per second or the energy per discharge.
The EDM process has numerous applications such as machining cavities and dies, cutting small-diameter holes, blanking parts from sheets, cutting off rods of materials with poor machinability and flat or form grinding.
For satisfactory EDM to take place, the spark must be completely quenched between charging cycles. If the spark is not completely quenched between charging cycles, arcing occurs (i.e. the spark is continuous). Arcing is most undesirable in that it produces gross surface damage to a substantial depth and also reduces the average efficiency of metal removal.
Numerous prior art patents have addressed the problems of arcing and the other problems associated with the critical mechanical and electrical parameters of the EDM process which must be controlled. For example, U.S. Pat. Nos. to Lobur 3,721,795, Syria et al 3,590,205 and Balleys et al 4,049,942 all disclose various electrical and mechanical mechanisms to control the EDM apparatus.
Similarly, the U.S. Pat. Nos. of Verner et al 3,697,719, Lobur 3,749,877, Sennowitz 3,761,673 and Wohlabaugh 3,843,864 all deal specifically with the current arcing problem.
The U.S. Pats. Nos. to Holliday 3,697,879, Sennowitz 3,808,392 and Wohlabaugh 3,996,445 all deal with the generation of timing pulses utilized in the EDM process.
The U.S. Pat. No. to Wohlabaugh 3,887,782 discloses the use of a vibrating electrode for flushing of the dielectric.